1. Field of the Invention
The present invention relates to a class AB amplifier output stage and, more particularly, to a current mirror and output driver for such an output stage.
2. Description of the Related Art
An output stage of an amplifier must satisfy many performance specifications, such as delivering a specified signal power to a load while maintaining a low signal distortion. It is also desired that the output stage limit output impedance so that voltage gain does not vary greatly when the load impedance changes. The output stage should consume low quiescent power and must not greatly limit the frequency response of the amplifier.
FIG. 1 illustrates a schematic circuit diagram of an output stage of an operational amplifier (op-amp) in accordance with U.S. Pat. No. 4,570,128 (Monticelli D. M.) "Class AB Output Circuit with Large Swing," issued on Feb. 11, 1986. Current source 150 causes current i.sub.1 flow in transistors 136 and 138. Current source 152 causes current i.sub.2 flow in transistors 132 and 134. Transistors are configured as a current mirror so that quiescent current flow in transistor 122 is defined by current flowing in the circuit formed by transistors 128, 130, 136 and 138. A similar configuration of transistors as a current mirror is furnished by transistors 124, 126, 132 and 134 to define the current flowing in output transistor 120. These transistors are configured so that the current through mirrored transistors is set by emitter area ratios and current densities in the emitters, so that the bias or quiescent currents are relatively stable over variable temperature and process conditions. Assuming that the base current of the transistors is negligible, transistor 138 drives a desired current i.sub.6 flow in transistor 130. Current i.sub.6 is partially supplied from each of current i.sub.7 flow in transistor 140, current i.sub.4 flow in transistor 126 and current i.sub.5 flow in transistor 128. Current i.sub.5 and i.sub.6 are supplied from current i.sub.3 flow in transistor 124, which is a mirror driven by transistor 132 current i.sub.2 flow. By virtue of the current mirrors, all currents are predetermined so that the currents i.sub.8 and i.sub.9 are equal and quiescent current at terminal 114 is negligible.
However, the assumption of negligible base current is greatly optimistic when temperature, operating frequency and process variations of the transistors are considered, particularly in the case of pnp transistors. Bipolar pnp devices are generally inferior to npn devices in frequency response and high-current behavior. Current gain .beta..sub.F is assumed to remain constant during operation of a circuit. However, .beta..sub.F does vary with operating conditions. For example, increasing the value of V.sub.CE increases I.sub.C while there is little change in I.sub.B. Therefore, the effective .beta..sub.F increases. Also, .beta..sub.F varies both with temperature and transistor collector current. Variations with operating conditions cause greater changes in .beta..sub.F for pnp transistors than for npn transistors. Parameters of a transistor, particularly .beta..sub.F, are subject to process variation.
In the actual case, current flows from the base of transistor 124 and the base of transistor 126, reducing the current i.sub.2 flow in transistor 132. The mirrored current i.sub.3 flow in transistor 124 is thus reduced. Since the sum of currents i.sub.3 and i.sub.7 drive output transistor 122, a reduction in current i.sub.3 places a greater demand on emitter follower 140 to drive the output of the output stage 100. In addition, over temperature and process variations this base current can change dramatically and the change in the base current alters the bias current i.sub.7 flow in transistor 140. Thus the temperature and process variations reflect directly back to the input stage of the op-amp as an offset error.